Heffernan et al. 2019, minerals and trace elements in athletic performance

Nutrients systematic review of mineral and trace-element supplementation (including magnesium) and exercise performance. For magnesium the results were mixed and inconsistent in athletes who are not deficient: no evidence that supplementation enhances endurance capacity, and only modest, status-dependent signals for some strength and dynamic-power measures, mostly in untrained people. Effects depend on baseline magnesium status, dose and population; the evidence base was rated moderate overall, with few strong-quality studies. The practical implication is that supplementing on top of an adequate intake does little, while correcting a genuine deficiency is the real lever.